This invention relates to magnetic field, electric current, and frequency sensing, and more particularly to a high-sensitivity, non-contact magnetic field, direct current, frequency, and alternating current detector.
Non-contacting AC current detectors are known in the art but suffer from some significant draw-backs. These detectors do not readily detect low values of DC current without invading the circuit or, if done magnetically, by first calibrating the instrument for a particular location to correct for the Earth""s or some other magnetic field or interference. Furthermore, these devices tend to be large and very expensive.
Because of these problems, the least expensive and bulky method of determining whether DC current is flowing through a conductor is by interrupting the circuit in order to connect an ammeter to the conductor as part of the circuit. This method creates its own problems. For example, when trying to locate a shortxe2x80x94e.g., to groundxe2x80x94in a circuit, it requires multiple tests along the length of the circuit.
Non-contact DC current detectors typically sense the magnetic field created by a DC current. The most common method to do this is by use of a Hall-effect sensor. Hall effect sensors are relatively small and inexpensive. Previously, Hall effect sensors typically generated only a very small raw signal of approximately five millivolts in a magnetic field on the order of one hundred gauss. As a result, they were ineffective at sensing small magnetic fields and were usually used in connection with permanent magnets. Recent improvements in Hall effect sensors have allowed the sensors to generate signals in the range of two to ten millivolts in a magnetic field on the order of one gauss. Hall effect sensors do not, by themselves, give an output linear to the magnetic field in which the sensor is placed. However, the devices are usually sold with associated electronics which convert the output into a linear function relative to the surrounding magnetic field. Thus, the commercially available products incorporating Hall-effect sensors usually do have linear outputs relative to their surrounding magnetic field. Specifically, a list of such commercially available Hall effect sensors would include: MLX90215 Precision Programmable Linear Hall Effect Sensor, Datasheet revision 2.1 (Apr. 10, 1998), Melexis Microelectronic Integrated System, 15 Sutton Road, Box 837, Webster, Mass. 01570-0837; A3515LUA Ratiometric, Linear Hall-Effect Sensors for High Temperature Operation, Datasheet 27501.10, Allegro MicroSystems, Inc., 115 Northeast Cutoff, Box 15036, Worcester, Mass., 01615-0036; F. W. Bell Hall Generators BH-200 Series, FH-301/FH-500 Series, GH-600/GH-700/GH-800 Series, BH-700 Series, BH-850, BH-900 Series, Datasheet, F.W. Bell Corp., 6120 Hanging Moss Road, Orlando, Fla., 32807, (407) 678-6900; Asahi Hall Elements HW-101/HW-104/HW-105/HW-108/HW-109/HW-300/HW-302/HW-305/HZ-106C/-HZ302C/HZ302H/HG-106C, Datasheet, Asahi Kasei Electronics Co., Ltd., 1-1-1 Uchisaiwai-Cho, Chiyoda-Ku, Tokyo 100, Japan.
Anisotropic magnetoresistive (AMR) sensors are also available and can sense low-strength magnetic fields. Examples of anisotropic magnetoresistive sensors include: Honeywell HMC1001 and HMC1002 One and Two Axis Magnetic Sensors, Datasheet 900150, Rev. E, 12/97, Honeywell, Inc., Solid State Electronics Center, 12001 State Highway 55, Plymouth, Minn. 55441, (800) 323-8295; Honeywell HMC/HMR Series, Datasheet 900187, 10/96, Honeywell, Inc., a Solid State Electronics Center, 12001 State Highway 55, Plymouth, Minn. 55441, (800) 323-8295.
The giant magnetoresistive (GMR) sensor has been available for some time. An example of a GMR sensor includes a resistive bridge circuit of four magnetoresistive devices connected between voltage bias input terminals and difference output terminals. Two legs of the resistive bridge circuit are positioned within the magnetic environment and two legs of the bridge circuit are shielded from the magnetic environment. The resistive bridge circuit experiences a change in electrical resistance of the two legs that are positioned within the magnetic environment in response to a change in the bridge circuit""s magnetic environment.
At the sensitivity levels exhibited by the GMR, AMR, or improved Hall effect sensors, magnetic fields generated by relatively low-current electrical and electronic devices can be detected. Because of this sensitivity, a GMR sensor or a Hall effect sensor, like an AMR sensor, can detect the Earth""s own magnetic field. However, the strengths of these gross fields and the fields sought to be detected by an AMR, GMR or Hall effect sensor may be of the same order of magnitude. The AMR, GMR or Hall effect sensors cannot reliably detect, much less easily measure low DC currents without eliminating the effects of the earth and other, gross magnetic fields.
U.S. Pat. No. 4,639,674, granted to Rippingale on Jan. 27, 1987, discloses electromagnets wrapped around magnetic cores (col. 2, In. 65 though col. 3 In. 20) as the sensing elements. Rippingale thereby teaches away from the use of magnetoresistive components as the way to follow conductors that have been energized with a varying electrical current signal. This prior art has difficulty separating and eliminating the signals the user wants to detect from the ambient noise signals.
U.S. Pat. No. 3,991,363, granted to Lathrop on Nov. 9, 1976, also discloses the use of coils to sense magnetic fields, thus teaching away from the use of magnetoresistive devices. The use of coils can present difficulties in designing devices which require greater sensitivity with less bulk in the sensor. Additionally, Lathrop calls for finding electrical leakage by making electrical (ohmic) contact with the conductive ground return conductor path.
U.S. Pat. No. 5,041,780, granted to Rippel on Aug. 20, 1991, discloses the use of two oppositely-polarized magnetoresistive magnetic flux sensors to cancel the extraneous magnetic environment when sensing both sides of the circular magnetic field around an electrical conductor. This device uses a simple amplifier and meter to display the flux level to the operator.
The purpose of this invention is to provide a non-contacting method of sensing an electromagnetic field and further allows for sensing, in a non-contact manner, low-level electrical currents by placing sensors near or in opposing positions on either side of a field conductor to be sensed, and adding the output algebraically. This allows common field effects to be canceled and the device to sense only that, field generated within the sensing zone.
In accordance with the present invention, a probe comprising at least two sensor elements is combined with means for sensing a change in an electrical characteristic of the sensor environment, the sensors being arranged so that their electrical characteristics are sensed in a mutually opposite sense, with the electrical signals representing the sensor electrical characteristics being algebraically added to produce an output.
Also in accordance with the present invention, a probe comprising at least one sensor element is used to sense the presence of an electromagnetic field that varies in a periodic manner with changes in an electrical characteristic of the sensor being sensed only at said periodic rate.